1. Field of the Invention
This invention pertains to a carbon-to-liquids process which converts carbonaceous material to water, synthetic hydrocarbons, and oxygenates. The carbon-to-liquids process may employ one or more carbonaceous materials as feedstocks, and uses the produced oxygenates from product water to boost production of synthesis gas or hydrogen.
2. Background of the Invention
Generally, fuels produced from non-petroleum sources have been so expensive relative to fuels refined from crude oil that production has been very low. However, due to recent high prices for crude oil and due to significant increase in transportation fuels demand not met by current production from petroleum, there is a renewed interest in the development of economical processes for the conversion of various carbonaceous materials other than crude oil, such as for example coal, petroleum coke, tar sands, shale oil, natural gas and/or biomass. As used hereinafter, this type of conversion will be identified as ‘Carbon-to-Liquids’ conversion or ‘CTL’, regardless of what form (such as solid, liquid or gas) the carbonaceous feedstock may be.
One CTL process converts coal to synthetic hydrocarbons, and is generally known as a coal-to-liquids process. It has been known for several decades that coal can be converted to useful products by a large variety of gasification and liquefaction processes. Extensive research work has been done in connection with coal gasification and in connection with deep hydrogenation coal liquefaction in an attempt to produce liquid and gaseous fuels at reasonable cost. Two basic methods are used for converting solid carbonaceous material (e.g., coal) to liquid fuel. One method involves gasification of the coal and subsequent conversion of the generated gas to transportation fuels (e.g., gasoline), for example by the Fischer-Tropsch synthesis. The other method involves mixing dry pulverized coal particles with recycled solvent oil to produce a slurry and passing the slurry with hydrogen through a high-temperature high-pressure reactor to effect hydrogenation and hydrocracking. Almost all such processes for producing liquid fuel from coal include the manufacture of hydrogen or a mixture of hydrogen and carbon monoxide, also called ‘synthesis gas’ or ‘syngas’. Heretofore, the cost of producing the hydrogen from coal has been excessive, and the hydrogen produced has not been used in the most effective manner.
Another CTL process converts natural gas to synthetic hydrocarbons, and is generally known as a gas-to-liquids process or ‘GTL’. Large quantities of natural gas are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, large natural gas reserves have been found in remote areas where it is uneconomical to develop the reserves due to the lack of local markets for the gas and the high cost of transporting the gas to distant markets. This high cost is often related to the extremely low temperatures needed to liquefy the highly volatile gas during transport, a technology which yields a Liquefied Natural Gas or ‘LNG’. An alternative is to locally convert the natural gas to liquid higher boiling point hydrocarbon products that can be transported more cost effectively. Processes for converting natural gas to heavier hydrocarbon liquids are generally known in the art.
The CTL process generally involves two sequential steps for converting a carbonaceous feedstock to liquid products (e.g., synthetic transportation fuels). In the first step, carbonaceous feedstock is converted to form at least a mixture of carbon monoxide and hydrogen, also knows as ‘synthesis gas’ or ‘syngas’). In the second step employing the Fischer-Tropsch synthesis, carbon monoxide and hydrogen (formed from the first step) are converted into water and hydrocarbonaceous compounds. For example, the GTL process involves first reacting natural gas or methane, the major chemical component of natural gas, with oxygen and/or steam to form a mixture of carbon monoxide and hydrogen, and then employing the Fischer-Tropsch synthesis which converts synthesis gas into water and hydrocarbonaceous compounds. Cobalt, iron, ruthenium and/or nickel have been used as catalytic metal employed in catalysts used in Fischer-Tropsch synthesis for the production of diesel and/or gasoline fuels. Cobalt-based and iron-based catalysts are generally used for commercial-scale Fischer-Tropsch processes, in which production of hydrocarbons with five carbons or more (also called ‘C5+ hydrocarbons’) exceeds 25,000 barrels per day.
The hydrocarbonaceous compounds formed during the Fischer-Tropsch synthesis may include paraffins, olefins, and oxygen-containing compounds, also called ‘oxygenates’. Some of these hydrocarbonaceous compounds are water-soluble. Typically, the Fischer-Tropsch product stream contains hydrocarbonaceous compounds having a range of numbers of carbon atoms varying from 1 to 100 or more, and thus having a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield transportation fuels (e.g., gasoline, jet fuel, diesel fuel). Hydrocarbon waxes (which are highly paraffinic) may be subjected to a cracking step to break large wax molecules into smaller ones for conversion to liquid and/or gaseous hydrocarbons. Paraffins are particularly desirable as the basis of synthetic diesel fuel and/or jet fuel. In the production of a synthetic fuel using Fischer-Tropsch synthesis, it is generally desirable to maximize the production of high value liquid and/or wax hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
In addition to the hydrocarbon products of the Fischer-Tropsch process, product water is formed as shown in the following Equation (1):nCO+2nH2→CnH2n+nH2O.  (1)
The water production is quite significant because a mole of water is typically produced for every mole of carbon monoxide converted. The composition of the product water is largely dependent on the catalyst metal used in the Fischer-Tropsch process and the reaction conditions (e.g., temperature, pressure) employed. For non-shifting catalysts such as employing cobalt and/or ruthenium, the water gas shift reaction is minimal so the water production with non-shifting catalysts approaches that of the reaction stoichiometry. For shifting catalysts such as employing iron, the water gas shift reaction is more prominent, so the overall water production with shifting catalysts may be less than the reaction stoichiometry predicts.
The water produced from the Fischer-Tropsch synthesis generates at least one water source for the various aspects of the process according to the present invention and will be denoted in this specification by the term “product water.” Typically, the product water contains water-soluble and/or dispersed organic compounds. The organic compounds may include oxygenates such as aliphatic, aromatic and cyclic alcohols, aldehydes, ketones and organic acids, and to a lesser extent suspended non-oxygenated aliphatic, aromatic and cyclic hydrocarbons, such as olefins and paraffins. The Fischer-Tropsch product water may also contain small quantities of inorganic compounds including metals from the Fischer-Tropsch reactor, as well as nitrogen and sulfur-containing species that originate from the carbonaceous feedstock used in the first step of a CTL process. Typical organic contents in Fischer-Tropsch product water may be found for example in U.S. Pat. No. 7,150,831 by Dancuart Kohler et al and U.S. Pat. No. 7,153,432 by Kohler et al.
The product water from the Fischer-Tropsch synthesis is generally recovered from the gas effluent exiting the Fischer-Tropsch reactor. The recovery proceeds by passing the gas effluent through a separation unit. The separation may include condensing the gas effluent in a typical three-phase separator or in a series of hot and cold separators to generate three separate streams. The three streams exiting the separator(s) are: a tail gas, a hydrocarbon condensate including mainly hydrocarbons in the C5 to C20 range, and a product water stream. Additionally, a much smaller portion of the overall water production may be recovered from a liquid product stream exiting the Fischer-Tropsch reactor, by employing for example a separation technique such as filtration, settling, stripping and/or centrifugation.
The Fischer-Tropsch product water contains relatively high concentrations of oxygenates and other organic compounds, in particularly when originating from a high-temperature Fischer-Tropsch synthesis. Direct application or disposal of the Fischer-Tropsch product water is generally not feasible without further treatment to remove undesirable constituents. Environmental concerns prevent the disposal of product water derived from Fischer-Tropsch synthesis into natural water ways and the sea thereby presenting a case for the production and recovery of useable water at the source of the carbonaceous feedstocks.
In certain areas where carbonaceous feedstocks to the CTL plant are to be found and where the CTL plant is built, water may be in short supply and a relatively costly commodity. Because the product water may have little commercial value and freshwater may be in short supply within the vicinity of the CTL plant, the Fischer-Tropsch product water is usually purified for reuse within the CTL process, although a small portion of the purified water may be disposed off. The purification may entail removal of the dissolved and suspended organic compounds for example by biological treatment in a wastewater facility to generate the purified water. The degree of purification/treatment of the Fischer-Tropsch product water depends largely on the intended applications, and it is possible to produce a wide range of water qualities ranging from boiler feed water to partially treated water which may be suitable for discharge to the environment.
In addition, because the Fischer-Tropsch product water may contain a significant amount of organic compounds, some of the carbon value may be recovered by separating some of the organic components into different chemical stocks and/or solvents for potential sale to various niche markets. The recovery of chemical byproducts of commercial value from the product water may render the water purification process more economical. The revenues the sales generate and the expected savings from a substantially reduced load on a wastewater biological treatment plant may potentially offset the cost of recovering certain chemical byproducts (e.g., oxygenates dissolved in the product water). Drawbacks to this purification scheme however include high costs of building and operating a large wastewater treatment facility supplied with a high carbon load, and/or designing, building and operating a myriad of processes to recover from the product water certain byproducts (e.g., oxygenates and/or hydrocarbons) of particular high economic value. Indeed, these extra recoveries/separations must be tailored to the type of oxygenates generated by the operating conditions and the type of catalyst used in the Fischer-Tropsch synthesis. For example, a product water from a high-temperature Fischer-Tropsch synthesis contains a lot more dissolved oxygenates than a product water from a low-temperature Fischer-Tropsch synthesis. Or a product water from a low-temperature iron-based Fischer-Tropsch synthesis contains more dissolved and/or suspended olefins, oxygenates and aromatics than a product water from a low-temperature cobalt-based Fischer-Tropsch synthesis.
Beside issues regarding costs of product water purification and oxygenates separation/recovery, CTL processes, and particularly GTL processes, generally use hydrogen (H2) or hydrogen-rich gases for example for adjusting the hydrogen content of the syngas feed, for catalyst activation/regeneration and/or for upgrading of hydrocarbons products for example by hydroprocessing. However, hydrogen may be in short supply in CTL processes depending on the carbonaceous feedstock(s) and the type of syngas generators which are employed in the CTL plant. Furthermore, hydrogen is generally expensive to produce. The hydrogen can be produced by water hydrolysis, but it is more economically produced from a syngas generator which can provide a high hydrogen yield, such as a reforming reactor employing steam. The steam for such reforming reactor generally originates from natural sources of freshwater, such as from rivers or lakes. Drawbacks to using freshwater include the cost of acquiring (such as pumping) and processing the freshwater (such as removing particulates).
Thus for at least the foregoing reasons, there is a need for reducing the costs of purifying product water from the Fischer-Tropsch synthesis and/or other used water streams generated in a CTL plant in a wastewater treatment facility. In addition, there is a need for a more economical way to produce steam for steam-utilizing syngas generation. There is a further need for a more economical way to boost hydrogen production in a CTL plant. There is also a need to improve carbon utilization and recovery of water-soluble organic byproducts (e.g., oxygenates) for reuse in the overall CTL process. Further needs include more efficient ways for improving carbon efficiency and/or reusing the Fischer-Tropsch product water in lieu of treating this water with a sizeable carbon load at a large expense in a wastewater facility.